RFC 7590

Use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP)

Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 7590 &yet
Updates: 6120 T. Alkemade
Category: Standards Track June 2015
ISSN: 2070-1721
Use of Transport Layer Security (TLS) in the
Extensible Messaging and Presence Protocol (XMPP)
Abstract
This document provides recommendations for the use of Transport Layer
Security (TLS) in the Extensible Messaging and Presence Protocol
(XMPP). This document updates RFC 6120.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7590.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

3. Recommendations
The best current practices documented in the "Recommendations for
Secure Use of TLS and DTLS" [RFC7525] are included here by reference.
Instead of repeating those recommendations here, this document mostly
provides supplementary information regarding secure implementation
and deployment of XMPP technologies.
3.1. Support for TLS
Support for TLS (specifically, the XMPP profile of STARTTLS) is
mandatory for XMPP implementations, as already specified in [RFC6120]
and its predecessor [RFC3920].
The server (i.e., the XMPP receiving entity) to which a client or
peer server (i.e., the XMPP initiating entity) connects might not
offer a stream feature of <starttls xmlns='urn:ietf:params:xml:ns
:xmpp-tls'/>. Although in general this stream feature indicates that
the server supports and offers TLS, this stream feature might be
stripped out by an attacker (see Section 2.1 of [RFC7457]).
Similarly, the <required/> child element of the <starttls/> stream
feature is used to indicate that negotiation of TLS is mandatory;
however, this could also be stripped out by an attacker. Therefore,
the initiating entity MUST NOT be deterred from attempting TLS
negotiation even if the receiving entity does not advertise support
for TLS. Instead, the initiating entity SHOULD (based on local
policy) proceed with the stream negotiation and attempt to negotiate
TLS.
3.2. Compression
XMPP supports an application-layer compression technology [XEP-0138].
Although this XMPP extension might have slightly stronger security
properties than TLS-layer compression (since it is enabled after
Simple Authentication and Security Layer (SASL) authentication, as
described in [XEP-0170]), this document neither encourages nor
discourages use of XMPP-layer compression.
3.3. Session Resumption
To improve the reliability of communications over XMPP, it is common
practice for clients and servers to implement the stream management
extension [XEP-0198]. Although that specification includes a method
for resumption of XMPP streams at the application layer, also using
session resumption at the TLS layer further optimizes the overall
process of resuming an XMPP session (see [XEP-0198] for detailed
information). Whether or not XEP-0198 is used for application-layer

session resumption, implementations MUST follow the recommendations
provided in [RFC7525] regarding TLS-layer session resumption.
3.4. Authenticated Connections
Both the core XMPP specification [RFC6120] and the CertID
specification [RFC6125] provide recommendations and requirements for
certificate validation in the context of authenticated connections.
This document does not supersede those specifications (e.g., it does
not modify the recommendations in [RFC6120] regarding the Subject
Alternative Names or other certificate details that need to be
supported for authentication of XMPP connections using PKIX
certificates).
Wherever possible, it is best to prefer authenticated connections
(along with SASL [RFC4422]), as already stated in the core XMPP
specification [RFC6120]. In particular:
o Clients MUST authenticate servers.
o Servers MUST authenticate clients.
o Servers SHOULD authenticate other servers.
This document does not mandate that servers need to authenticate peer
servers, although such authentication is strongly preferred.
Unfortunately, in multi-tenanted environments it can be extremely
difficult to obtain and deploy PKIX certificates with the proper
Subject Alternative Names (see [XMPP-DNA] and [PKIX-POSH] for
details). To overcome that difficulty, the Domain Name Associations
(DNAs) specification [XMPP-DNA] describes a framework for XMPP server
authentication methods, which include not only PKIX but also DNS-
Based Authentication of Named Entities (DANE) as defined in
[DANE-SRV] and PKIX over Secure HTTP (POSH) as defined in
[PKIX-POSH]. These methods can provide a basis for server identity
verification when appropriate PKIX certificates cannot be obtained
and deployed.
Given the pervasiveness of eavesdropping [RFC7258], even an encrypted
but unauthenticated connection might be better than an unencrypted
connection in these scenarios (this is similar to the "better-than-
nothing security" approach for IPsec [RFC5386]). Encrypted but
unauthenticated connections include connections negotiated using
anonymous Diffie-Hellman mechanisms or using self-signed
certificates, among others. In particular for XMPP server-to-server
interactions, it can be reasonable for XMPP server implementations to
accept encrypted but unauthenticated connections when Server Dialback
keys [XEP-0220] are used; such keys on their own provide only weak

identity verification (made stronger through the use of DNSSEC
[RFC4033]), but this at least enables encryption of server-to-server
connections. The DNA prooftypes mentioned above are intended to
mitigate the residual need for encrypted but unauthenticated
connections in these scenarios.
3.5. Server Name Indication
Although there is no harm in supporting the TLS Server Name
Indication (SNI) extension [RFC6066], this is not necessary since the
same function is served in XMPP by the 'to' address of the initial
stream header as explained in Section 4.7.2 of [RFC6120].
3.6. Human Factors
It is strongly encouraged that XMPP clients provide ways for end
users (and that XMPP servers provide ways for administrators) to
complete the following tasks:
o Determine if a given incoming or outgoing XML stream is encrypted
using TLS.
o Determine the version of TLS used for encryption of a given
stream.
o If authenticated encryption is used, determine how the connection
was authenticated or verified (e.g., via PKI, DANE, POSH, or
Server Dialback).
o Inspect the certificate offered by an XMPP server.
o Determine the cipher suite used to encrypt a connection.
o Be warned if the certificate changes for a given server.
4. Security Considerations
The use of TLS can help to limit the information available for
correlation between the XMPP application layer and the underlying
network and transport layers. As typically deployed, XMPP
technologies do not leave application-layer routing data (such as
XMPP 'to' and 'from' addresses) at rest on intermediate systems,
since there is only one hop between any two given XMPP servers. As a
result, encrypting all hops (sender's client to sender's server,
sender's server to recipient's server, and recipient's server to
recipient's client) can help to limit the amount of metadata that
might leak.

It is possible that XMPP servers themselves might be compromised. In
that case, per-hop encryption would not protect XMPP communications,
and even end-to-end encryption of (parts of) XMPP stanza payloads
would leave addressing information and XMPP roster data in the clear.
By the same token, it is possible that XMPP clients (or the end-user
devices on which such clients are installed) could also be
compromised, leaving users utterly at the mercy of an adversary.
This document and related actions to strengthen the security of the
XMPP network are based on the assumption that XMPP servers and
clients have not been subject to widespread compromise. If this
assumption is valid, then ubiquitous use of per-hop TLS channel
encryption and more significant deployment of end-to-end object
encryption technologies will serve to protect XMPP communications to
a measurable degree, compared to the alternatives.
This document covers only communication over the XMPP network and
does not take into account gateways to non-XMPP networks. As an
example, for security considerations related to gateways between XMPP
and the Session Initiation Protocol (SIP), see [RFC7247] and
[RFC7572].
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <http://www.rfc-editor.org/info/rfc6120>.

Appendix A. Implementation Notes
Some governments enforce legislation prohibiting the export of strong
cryptographic technologies. Nothing in this document ought to be
taken as advice to violate such prohibitions.
Acknowledgements
The authors would like to thank the following individuals for their
input: Dave Cridland, Philipp Hancke, Olle Johansson, Steve Kille,
Tobias Markmann, Matt Miller, and Rene Treffer.
Roni Even caught several important issues in his review on behalf of
the General Area Review Team.
Ben Campbell, Spencer Dawkins, and Barry Leiba provided helpful input
during IESG review.
Thanks to Leif Johansson and Orit Levin as chairs of the UTA WG, Ben
Campbell and Joe Hildebrand as chairs of the XMPP WG, and Stephen
Farrell as the sponsoring Area Director.
Authors' Addresses
Peter Saint-Andre
&yet
EMail: peter@andyet.com
URI: https://andyet.com/
Thijs Alkemade
EMail: me@thijsalkema.de